1. Field of the Invention
The present invention relates generally to processes for semiconductor manufacturing and more particularly to the area of optical lithography.
2. Description of Related Art
Reductions in the size of semiconductor chips requires a proportional tightening of lithographic projection machine (machine) performance and a corresponding improvements in variance from machine to machine and across the machine projection field. See, for example, “International Technology Roadmap for Semiconductors”, 2001 Edition, Executive Summary; “International Technology Roadmap for Semiconductors”, 2001 Edition, Front End Processes; “International Technology Roadmap for Semiconductors”, 2001 Edition, Lithography, “International Technology Roadmap for Semiconductors”, 2001 Edition, Metrology; “International Technology Roadmap for Semiconductors”, 2001 Edition, Modeling and Simulation, “International Technology Roadmap for Semiconductors”, 2001 Edition, Yield Enhancement.
Presently lithographers adjust the properties of the illumination source (partial coherence, annularity, etc.) to increase the useable processing window. See, for example, “High Throughput Wafer Steppers with Automatically Adjustable Conventional and Annular Illumination Modes”, J. Mulkens et al. As used herein, “illumination source” means the collective effect of the pre-reticle optics (such as mirrors, homogenators, lenses, polarizers, diffusers, etc.) and the light source (mercury arc lamp, excimer laser, synchrotron radiation, etc.) on creating a radiant intensity pattern (energy per unit solid angle) at the reticle. For Kohler Illumination (see, for example, “Principles of Optics”, M. Born et al., Pergamon Press, 524:526), the source on a particular machine, and for a particular machine setting, is completely characterized by the radiant intensity given by:
                                                        ⅆ              E                                      ⅆ              o                                ⁢                      (                                          n                ⁢                                                                  ⁢                x                            ,                                                n                  ⁢                                                                          ⁢                  y                                ;                x                            ,              y                        )                          =                  energy  per  unit  solid  angle  coming  from  direction  (          nx          ,          ny          )and  at  transverse  spatialposition  (          x          ,          y          )  on  the  reticle.                                    (                  Equation          ⁢                                          ⁢          1                )            
The ability to predict lithographic performance, especially cross-field or machine to machine variation, is contingent on quantitatively knowing the factors causing variation and this includes the illumination source
      (                            ⅆ          E                          ⅆ          o                    ⁢                          ⁢      of      ⁢                          ⁢      Equation      ⁢                          ⁢      1        )    .The effect of the illumination source (source) when coupled to projection imaging objective (PIO, or lens that relay the reticle object plane to the wafer plane) aberrations has been documented, as has the deleterious effects of improperly or non-optimally configured sources themselves on lithographic printing. See, for example, “Differences of Pattern Displacement Error Under Different Illumination Conditions”, N. Seong et al., SPIE, Vol. 3334, 868:872, 1998; “Effect of Off-Axis Illumination on Stepper Overlay”, N. Farrar, SPIE, Vol. 2439, 273:275, 1995; “Overlay Error Due to Lens Coma and Asymmetric Illumination Dependence”, H. Nomura et al., SPIE, Vol. 3332, 199:210, 1998; and see “The Effects of an Incorrect Condenser Lens Setup on Reduction Lens Printing Capabilities”, D. Peters, Interface 85, Kodak Publ. No. G-154, 66:72, 1985; “Impact of Local Partial Coherence Variations on Exposure Tool Performance”, Y. Borodovsky, SPIE, Vol. 2440, 750:770, 1995; “Condenser Aberrations in Kohler Illumination”, D. Goodman et al., SPIE, Vol. 922, 108:134, 1988; “Mathematical Treatment of Condenser Aberrations and their Impact on Linewidth Control”, C. Krautschik et al., Intel, 1:12, 1998; “Examples of Illumination Source Effects on Imaging Performance”, A. J. deRuyter et al., ARCH Chemicals Microlithography Symposium, 2003. Comprehensive modeling will generally require knowing the radiant intensity across the projection field, machine settings, and machines. See, for example, “Understanding Systematic and Random CD Variations using Predictive Modeling Techniques”, D. Flagello et al., SPIE, Vol. 3679, 162:175, March 1999; “Understanding Across Chip Line Width Variation: The First Step Toward Optical Proximity Correction”, L. Liebmann et al., SPIE, Vol. 3051, 124:136, 1997.
Typically, a lithographer will have been provided the nominal value or interpretation of each illumination setting by the machine manufacturer, and this is useful for lowest order process window determination. This is insufficient for dealing with and characterizing observed variations, for this field point and machine dependent radiant intensity is usually required. See, for example, “Examples of Illumination Source Effects on Imaging Performance”, supra.
In-situ source measurement techniques have been previously described. See, for example, “Pinholes and Pupil Fills”, J. Kirk et al., Microlithography World Autumn 1997, 25:28, 1997; “Impact of Local Partial Coherence Variations on Exposure Tool Performance”, supra; “In-Situ Source Metrology Instrument and Method of Use”, A. Smith et al., U.S. Pat. No. 6,356,345 issued Mar. 12, 2002. The continued drive to reduce semiconductor size has made it increasingly difficult to observe and characterize the properties of the illumination source in-situ. Therefore, the need to accurately measure high resolution illumination sources in-situ in projection imaging systems remains. It is thus advantageous to have an apparatus and method for rapid and accurate high resolution characterization of sources.